1. Field of the Invention
This invention relates in general to valves for the control of gas or liquid fluids. More particularly, the invention relates to miniature valves such as microvalves which are electrically actuated using the shape memory properties of certain alloy thin films.
2. Description of the Prior Art
The prior art includes the Busch et al. U.S. Pat. No. 5,061,914 entitled Shape-Memory Alloy Micro-Actuator which is assigned to TiNi Alloy Company, the assignee of the present invention. The Busch et al. patent describes initial work performed by the inventors which made thin film shape memory alloys possible. The patent discloses a method for producing small actuator mechanisms using thin film shape memory alloys, specifically for moving elements with respect to one another.
In the prior art the miniaturization and consolidation of electronics and mechanisms has become a necessary characteristic of technological progress. Although the electronics industry has been very successful in producing integrated circuits and microprocessors on a micrometer scale, mechanical devices must also follow suit. One immediate need for millimeter size mechanisms is in the field of fluid flow control. Extremely small valves and pumps are desired in such fields as gas and liquid chromatography, biomedical research, medical instruments, robotics, building HVAC systems and factory automation equipment. The present invention provides a new design which meets many of these commercial needs.
The smallest electrically operated and commercially available valves, until recently, were solenoid-driven devices occupying several cubic centimeters. Scaling to smaller dimensions by means of solenoids, or electromagnetic technology in general, is highly unlikely because of the difficulty in obtaining sufficient actuation force in scale-down as well as in the complexities of manufacturing. New actuation means are being developed in the prior art to make millimeter size microvalves feasible.
One such approach described in J. H. Jerman "Electrically-Activated, Normally-Closed Diaphragm Valves," International Conference on Solid-State Sensors and Actuators (IEEE #91CH 2817-5), Montreux, Switzerland, Jun. 24-27, 1991 and J. H. Jerman, "Electrically-Activated Micromachined Diaphragm Valves, Technical Digest" IEEE Solid-State Sensors and Activator Workshop, Hilton Head, N.C., USA, pp. 65-69, 1990 uses a silicon membrane as a poppet, but it is actuated by means of differential thermal expansion. Thin film resistors are embedded onto the surface of a bimetallic membrane. When the membrane is heated, it deflects toward or away from a valve seat. Such a valve approach has many of the same disadvantages described above. Thus, the stroke of the membrane is severely limited, thereby diminishing its tolerance for small particles in the flow stream. Although this design does not have as much mass-to-heat ratio, it still has a relatively slow time constant. Fabrication of the actuator is also complicated, requiring silicon, metal, resistor implantation and circuitry.
Another approach is similar to a switch actuator suggested by U.S. Pat. No. 4,864,824. In this third approach, a membrane is deformed by air against a valve seat.
A third approach, described in U.S. Pat. Nos. 4,943,032, 4,824,073 and 4,821,997 takes advantage of special fluids which expand upon heating. The expansion is used to deform a membrane toward an opposing valve seat, thereby closing the valve. These devices have several disadvantages. First, the fluid must be hermetically sealed in a cavity behind the working membrane. Any leaks or change in cavity volume will reduce the performance of the valve. Second, because these valves are fabricated in silicon, which is the method of choice for microdevices, the working membrane must also be made of material with limited elastic strain capability. This means that the valve displacement, limited by a tolerable elastic strain of approximately 0.1%, is very small relative to the membrane diameter. To achieve sufficient "poppet" stroke, the valve assembly must be relatively large. Further, the inherently limited distance between membrane and valve seat diminishes the valve's ability to handle flow streams that contain small particles. Third, the time constant for such a valve would be very long because the heat used to expand the fluid must be dissipated before the next cycle can begin. This problem is exacerbated by heat capacity of the working fluid. Fourth, such a design enables only a normally open valve. Most commercial applications prefer normally closed valves.
The need has therefore been recognized for a microvalve which obviates many of the disadvantages of the prior art devices, and in particular which operates with lower power consumption, faster response time, is capable of linear proportional control and higher flow rates, and which is simple in construction and can be easily manufactured.